1. Field of the Invention
The present invention relates to a scanning tunneling microscope (STM).
2. Description of the Related Art
An attempt has been made to simultaneously measure the optical characteristic and STM image of the surface of a sample and obtain the data relating to the electronic state, lattice vibration, etc. of the sample surface, for example, the structure and energy state density.
For this purpose, an optical microscope-integrated STM, for example, has been proposed. In this STM, a probe is attached to a transparent plate, and the center axis of the probe is made to agree with the optical axis of a sample observation optical system. Thus, the sample surface can be observed by the STM while the sample surface can be optically observed through the transparent plate. In addition, if a spectroscope is provided, the optical characteristics of the sample surface, e.g. absorption, reflection and light emission, can be measured.
In the optical microscope-integrated STM, however, when the optical characteristics of the sample surface are analyzed by using the spectroscope or the like to enhance the wavelength resolution, only average optical characteristics of the entire optical visual field ar measured. Thus, the horizontal resolution of the optical image decreases remarkably. In addition, when the sample surface is optically excited by light emitted from an objective lens and a resultant tunnel current is detected, it is not possible to optically excite only the sample surface (in a range of about 10 nm.sup.2) near the region where tunnel current is detected.
There has been proposed another apparatus, a near-field optical-scanning microscope (NFOSM). The NFOSM is described in detail in "J. Appl. Phys.," Vol. 59, No. 10, 15 May 1986, pp. 3318-3327. FIG. 3 of this document illustrates the principle of this apparatus. In FIG. 3, a crystal tip has one end portion with a radius of curvature of 30 nm, and this end of the tip is provided with a light-transmission hole. The hole is formed by coating the tip with Cr, Ag or Al and then with Au and thereafter pressing the tip on a glass surface When a laser beam is introduced from the other end portion of the tip, the beam is diffracted by the light-transmission hole and is radiated outward while flaring. According to the NFOSM, the sample and the transmission hole are sufficiently approached and the sample is observed by utilizing the diffracted light. Compared to an optical microscope, a higher horizontal resolution (.about..lambda./20) is obtained. In the NFOSM, an STM is used for positional control in the z-direction perpendicular to the sample surface. Accordingly, using an STM an optical image with high horizontal resolution (absorption, reflection, light emission) and an STM image can be obtained simultaneously.
When optical measurement and STM measurement of the sample surface are carried out by use of the NFOSM, light is radiated from the light-transmission hole at the end portion of the tip, as shown in FIG. 5 of the aforementioned document, while a tunnel current flows through an Au film near the light-transmission hole. Thus, the location for detecting the optical characteristic does not coincide with the location for detecting the STM image. Consequently, the location where light excitation takes place differs from the location where tunnel current is detected, and it is almost impossible to detect a light-excited tunnel current.